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Health officials in Leeds, Grenville and Lanark are warning people with homes or cottages off the Upper Rideau Lake's McNally's Bay not to drink water from the bay after toxins were discovered.

Residents with homes off McNally's Bay — about 40 kilometres south of Perth, Ont., — had contacted health officials after seeing blue-green algae blooms in the lake.

As a precaution, the Leeds, Grenville and Lanark health unit mailed residents on Monday warning them of the potential for toxins.

Ontario's Ministry of Environment confirmed their presence on Thursday.

Health officials said that boiling water does not remove the toxins and also said that pets and livestock should not drink directly from the lake.

Health officials say blue-green algae can range in colour from olive-green to red and can cause the pond scum sometimes seen in shallow, slow-moving water. (Megan Thomas/CBC)

Blue-green algae toxins, when found in high enough amounts, can cause headaches, fever, diarrhea, abdominal pain, nausea or vomiting if ingested. Skin contact can also cause rashes and mucous membrane irritation.

Residents said they suspect partially treated sewage from the nearby community of Westport could be to blame, as could heavy rainfall washing fertilizer into the water.

Ministry of Environment and local health officials are collecting additional water samples to further assess the risk, but are also cautioning residents to take precautions in and around algae blooms, particularly when swimming.

Residents who see blue-green algae blooms are asked to contact the Spills Action Centre at 1-800-268-6060, while people can also get further updates on their website or by calling them at 1-800-660-5853.